Experimental study of local scour around submerged compound piles in steady current

Many subsea structures have non-uniform geometries, which extend only a finite distance above the seabed. To better understand local scour at these structures, this paper presents scour measurements from an experimental study of submerged compound piles comprised of a circular superstructure supported by a circular foundation. The experiments were conducted in the clear-water regime and considered the effect of three different ratios describing the structural geometry: (i) the overall height of the compound structure above the initial bed level to the diameter of the super-structure h₁/D₁; (ii) the height of the foundation exposed above the initial bed level relative to the overall height of the superstructure h₂/h₁; and (iii) the diameter of the foundation to that of the superstructure D₂/D₁. In all cases D₂≥D₁ and h₁≥h₂≥0 (i.e. the foundation and superstructure where never buried). The experimental results indicate that, if all other parameters are held constant, the non-dimensional equilibrium scour depth S₀/D₂ increases with both h₁/D₁ and h₂/h₁, and decreases as D₂/D₁ increases. To quantitatively explain these trends the experimental results are compared with a range of existing methods to predict scour depth for non-uniform (or complex) surface piercing piles. Of these methods it is found that an effective pile height and an effective pile diameter, computed using simple geometric arguments, give good agreement. The concept of effective height is also shown to collapse time-scale measurements of the scour process convincingly across the full range of experimental results and to explain the extent and shape of the local scour hole reasonably well. Collectively, these findings suggest that the calculation of an effective height may be used to provide a reasonable first approximation to estimate the rate and extent of local scour around submerged compound structures.